1. Introduction

Since the first demonstration of the passively modelocked, femtosecond Ti:sapphire oscillator in the early nineties [1], this laser has found countless applications and a number of its distinct variations have been developed. Among these there are the two extremes in terms of the pulse repetition rate governed by the laser cavity length. One reaches towards long cavities (extended with the q-preserving Herriott cells [2], typically up to several meters) and thus increases the pulse energy up to the 100-nJ level [3] at the expense of a low repetition rate. At the other extreme the high repetition rate designs [4, 5] are now reaching ten gigahertz with minute cavity layouts [6]. The latter has become increasingly important with the advent of the femtosecond laser based optical frequency comb technology developed at the turn of the century [7, 8]. Gigahertz laser based optical frequency combs [9, 10] not only outperform the early designs in compactness, better mechanical stability and easier access to the individual comb frequencies in the preliminary characterization but, more importantly, provide a significantly higher signal-to-noise ratio in the homodyne beats measurement simply since for a given average power of the laser the optical power per comb mode is proportional to the repetition rate.

Apart from the Ti:sapphire, only a few other lasers has been demonstrated to operate in the high repetition rate, passively modelocked configuration: Nd:YVO[11], Cr:YAG[12, 13], Cr:forsterite[14] and Er:Yb:glass[15].

The choice of materials (either crystals, ceramics or glasses) that could replace the inefficient, bulky and expensive Ti:sapphire systems is limited by the availability of the high power laser diodes operating at the wavelengths matching the respective absorption bands. Ytterbium as a dopant element and the AlGaInAs laser diodes near 980 nm is one of the combinations that have emerged a few years ago [16, 17] and has proven to be a promising candidate for the next generation of compact, reliable femtosecond sources in applications where the sub-100 fs pulse durations is not required [18]. A number of host crystals has been used in passive, Kerr-lens modelocked, femtosecond lasers, among them KY(WO₄)₂ (KYW)[17], KGd(WO₄)₂ (KGW)[16], Sr₃Y(BO₃)₃ (BOYS)[19], CaF₂[20], CaGdAlO₄[21], Y2SiO₅ (YSO), Lu₂SiO₅ (LUSO)[22] and YVO₄[23] as well as YAG[24]. Most of the femtosecond laser sources based on the materials listed above were operated around 100 MHz pulse repetition rate, involving astigmatically compensated cavities with prism pair and/or chirped mirrors, commonly used in femtosecond lasers.

The marriage of the enabling Yb:crystal femtosecond laser technology and the laser frequency comb stabilization techniques first saw light only this year with the demonstration of the optical frequency comb with stabilized carrier-envelope offset based on a diode pumped Yb:KYW oscillator by Meyer, Squier and Diddams [25].

The gigahertz, diode pumped, high efficiency Yb:KYW oscillator demonstrated here offers an attractive alternative in the quest for compact and robust pulsed laser sources with high repetition rates. The basic design criteria were based on these of high repetition rate Ti:sapphire lasers [26] – small cavity mode waist in the laser crystal, tight pump focusing and a low transmission output coupler were used to obtain a high intensity in the crystal needed to support the mode locking by Kerr lensing.

2. Experimental setup

The laser schematics is presented in Fig. 1. The cavity is a four-mirror bow-tie ring resonator. Two concave mirror have the radii of curvature of 30 mm and the two other mirrors including the output coupler are flat on 30 min wedged substrates.

 

Fig. 1. Gigahertz Yb:KYW laser layout. LD - 980 nm, 500 mW single mode fiber coupled laser diode, CL - f=15 mm collimating lens, FL - f=30 mm focusing lens, PM - pump mirror (HT@980 nm and HR@1040±20 nm), X - 1.2 mm Yb:KYW crystal, -800 - negative dispersion mirror (-800 fs²@1030 nm), OC - output coupler. The laser cavity is drawn to scale. Inset shows the 0.65 GHz cavity with increased negative dispersion (see text for details).

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The cavity astigmatism was compensated in a standard manner by tilting the concave mirrors. The tilt angle (14.8 deg beam-to-beam) was calculated from the formula similar to the one given in [27]. We use the pump mirror at its center to avoid the transmitted pump beam deviation, while the other concave mirror was used near the edge to allow for more space between its edge and the laser beam.

We have also tested a slightly modified laser cavity design, shown in the inset of Fig. 1. Here both concave mirror were of the same type, and the cavity was folded twice to allow for introducing an additional -800 fs² chirped mirror. Due to mechanical constrains, the measured repetition rate was 0.65 GHz in this configuration. The laser produced around 60 mW of the output power (unidirectional) and after careful alignment the modelocking was self starting when the cavity was blocked and opened again.

The crystal was a commercially available (Crystals of Syberia) 1.2 mm, Brewster cut, KYW doped with Yb at 10 at.%. Pump polarization was parallel to the crystal N_m axis and the pump propagated along the N_p axis. The crystal had no active cooling.

To achieve optimal matching of the pump beam and the laser cavity mode we have chosen the single mode fiber coupled, single emiter laser diode (JDSU, 2900 series) that provided the maximum output power of 485 mW (in front of the crystal) at 980 nm ±0.5 nm . The fiber mode diameter is 4 μm and the fiber output beam was first collimated with an aspheric lens collimator of 15 mm focal length (AR coated, Thorlabs) and focused onto the Yb:KYW crystal with a 30 mm focal length plano-concave singlet (AR coated, CVI).The pump beam polarization direction was controlled by rotating the collimator in its mount.

All Yb:crystal, passively modelocked femtosecond lasers demonstrated up to date operate in the regime of a large negative group velocity dispersion. In our designs, the negative GVD was provided by chirped mirrors (Layertec) that, together with the Yb:KYW crystal, accounted for -1480 fs² (-2280 fs²) of the GVD per round trip in the 1 GHZ (0.65 GHz) cavity around 1040 nm (Yb:KYW crystal GVD calculated from the manufacturer data equals 200 fs²/mm).

3. Laser performance

When optimized for the CW operation, the laser provided maximum output power of 147 mW at around 1040 nm with 485 mW of the pump power incident on the crystal and the the slope efficiency was 36% (compare Fig. 2(a)). The laser cavity stability region spans over 3 mm in the concave M1 mirror position.

 

Fig. 2. Laser CW output power as a function of the pump power incident on the crystal a) and the measured modelocked laser spectrum b).

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To achieve modelocking, first the laser power is optimized in CW regime and then the position of the the mirror M1 is scanned in search for the regions where the laser spectrum bifurcates and the laser power becomes unstable. In some of those regions [28] one can achieve modelocking after a small, fast translation of the concave mirrors (M1) towards the crystal. The pulsed operation of the laser is stable even though no care was taken to isolate the laser from the environment.

When modelocked, the laser spectrum broadening is observed – the spectrum with 5.2 nm width, peaked at 1047 nm is presented in Fig. 2(b). The measured spectral width is sufficient to support pulses of approximately 200 fs duration. For other positions of mirror M1 we observed narrower (4.1 and 2.9 nm) spectra of similar shapes. This behavior is very similar to the one of our recently demonstrated Kerr lens modelocked femtosecond 82 MHz Yb:KYW oscillator [28]. The maximum modelocked output power was 115 mW which corresponds to 24% of the incident pump to optical output efficiency. The pulse trace measured with a fast InGaAs photodiode and a 1 GHz bandwidth, 20 Gsamples/s oscilloscope is presented in the inset of Fig. 3. It is worth noting that this measurement setup is used at its limits when recording the 1 GHz pulse repetition rate transient. With the same setup we have also verified that no long-scale pulse energy modulation is present which could indicate the Q-switched operation of the laser. Finally, the laser RF spectrum centered at 1.0138 GHz was recorded with the spectrum analyzer – see Fig. 3. We have verified that no structures characteristic of Q-switching were present in this spectrum either.

 

Fig. 3. RF spectrum of the laser output measured with 24 kHz RBW. Normalized laser output trace is shown in the inset.

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4. Conclusions and outlook

In conclusion, we have demonstrated what is to our knowledge the first passively modelocked, diode pumped, gigahertz-repetition rate Yb:crystal ring laser.

The presented high efficiency laser setup can be further improved by applying a two-side pumping with another pump diode and the same set of focusing optics. Given the output power reached with the current pump source, the laser should allow the supercontinuum generation in a microstructured photonic crystal fiber without external amplification of the pulses if the overall pump power is increased. With the carrier-envelope offset easily controlled by modulation of the pump diode power, this design is an excellent candidate for a simple and robust self-referenced optical frequency comb source.